The natural, or solitary, linewidth of a semiconductor laser is the linewidth the laser exhibits when it is operating with no external optical or electrical feedback present. Most single-spectral-mode semiconductor diode lasers that are readily available today have natural linewidths greater than a few megahertz. Typical values range from 5 MHz to 100 MHz. Narrow linewidth sources, however, are essential for optical systems such as coherent optical communications and optical sensors.
To accomplish linewidth narrowing in semiconductor lasers, it is necessary to couple light emitted from the laser back into the laser cavity. The front or rear facet emission has been used previously to couple reflected light back into the laser cavity. Utilization of the light from the front facet, as illustrated by the two examples in FIGS. 1A and 1B, either disturbs and/or attenuates the main beam of interest in the optical system. Utilization of the light from the rear facet, as illustrated by the two examples in FIGS. 2A and 2B, requires the laser to be mounted on a special narrow heat sink to allow unobstructed access to the rear facet emission. Most linewidth narrowing systems using front or rear emission have utilized either a Fabry-Perot semiconductor laser or a distributed feedback semiconductor laser as the active element. U.S. Pat. No. 4,955,028 discloses a wavelength tunable laser that uses an optical fiber coupled to the edge emission.
U.S. Pat. Nos. 5,109,386 and 5,111,467 disclose the use of vertical emission for stabilizing the wavelength of a distributed Bragg reflector laser using wavelength selective feedback to the laser by means of a rugate filter placed above the vertical emission.
The novelty of the present invention is the use of vertical emission for linewidth narrowing of DFB and DBR semiconductor lasers.